With the recent spread of portable electronic equipment such as notebook computers, video camcorders, and personal digital assistances, nonaqueous electrolyte secondary batteries having high voltage and high energy density have come to be widely used as a power source. From the concern for the environmental protection, electric-powered vehicles and hybrid-powered vehicles utilizing electric power as a part of motive power have already been put to practical use.
Various additives for nonaqueous electrolyte have been proposed to provide nonaqueous secondary batteries with improved stability or electrical characteristics. Examples of such additives include 1,3-propane sultone (see patent document 1 below), vinyl ethylene carbonate (see patent document 2 below), vinylene carbonate (see patent document 3 below), 1,3-propane sultone or butane sultone (see patent document 4 below), vinylene carbonate (see patent document 5 below), and vinyl ethylene carbonate (see patent document 6 below). Among them, vinylene carbonate is widely used for its high effectiveness. These additive compounds are considered to form a stable film called a solid electrolyte interface covering the surface of the negative electrode, which film is expected to prevent reductive decomposition of the electrolyte.
The recent rise in the price of rare metals, such as cobalt and nickel, has rapidly boosted use and development of a positive electrode active material containing a low-cost metal material, such as manganese or iron. A manganese-containing, lithium transition metal oxide salt is one of positive electrode active materials of current interest for its excellent performance, such as capacity and power, in nonaqueous secondary batteries. It is known, however, that manganese is liable to be eluted from a manganese-containing, lithium transition metal oxide salt as a positive electrode active material in a nonaqueous secondary battery particularly during storage in high temperatures or through repeated charge/discharge cycles. The eluted manganese induces a side reaction to cause deterioration of battery performance, such as an increase of internal resistance and reductions of capacity and power.
To prevent elution of manganese from a positive electrode, various additives for a nonaqueous electrolyte have been proposed, including a disulfonic ester as disclosed, e.g., in patent document 7 below. There has still been a demand for further improvements, though.